1. Field of the Invention
This invention relates in general to halftone image processing, and more particularly to a halftone method and apparatus that provides a line screen frequency of N/2 for a printer resolution of N without negative print effects.
2. Description of Related Art
As an approximation to continuous-tone images, pictorial imagery is represented via a halftone image processing apparatus and process in which each input pel is translated into a j×k pattern of recorded elements, where j and k are positive integers.
Herein, it will be understood that separate threshold matrices are used for each color component. However, as will become obvious from the discussion that follows, the color components will not be considered together. Rather, the term “pel” will be used to refer to “samples” in an array. A halftone image is reproduced by printing the respective pels or leaving them blank. That is, by suitably distributing the recorded elements.
Image processing apparatus and processes are evaluated in part, by their capability of delivering a complete gray scale at normal viewing distances. The capability of a particular process to reproduce high frequency renditions (fine detail) with high contrast modulation makes that procedure superior to one that reproduces such fine detail with lesser or no output contrast.
More recently, supercells have been developed by grouping multiple basic cells together. The supercell approach attempts to improve both gradation and resolution. This method is generally referred to as the “Improved Halftone” (IH) method. In the discussion that follows, the terms “dpi” (dots per inch) and “lpi” (lines per inch) are used throughout. Those skilled in the art will recognize that “dpi” refers to the printer resolution, which is the number of pels per linear inch. The dpi of a printer is linked to the size of each pel, wherein more dots per inch, the finer the resolution. Moreover, those skilled in the art will recognize that “lpi” refers to the halftone line screen resolution, which relates to the amount of lines a printer is able to print in one inch. The line screen frequency is typically defined as X lines/inch, where X=1/d and d is the shortest distance (in inches) between the closest imaginary lines drawn through the dot centers.
As an example of a super cell in the IH method, an 8×8 superthreshold value matrix may be divided into four 4×4 submatrices. The small matrix is adopted for the resolution unit, and the large matrix is adopted for the gradation unit. Thus, the amount of detail is determined by the basic cell size while the number of shades depends upon the supercell size. As the supercells have become larger, more than one pel inside the supercell is turned on for the next input level. However, never more than one pel is turned on at a time inside a basic cell for the next constant input level. The patterns generated by repetition of the original basic cell are contained in the patterns generated by the supercell.
Some newer printers are capable of printing additional levels between off and full on. For example, a high resolution printer may be able to print fourteen additional levels between off and full on. However, such a printer is not a true continuous-tone printer because large areas of solid intermediate values are badly mottled and unpleasant looking. Intermediate values may be printed as isolated spots or more reliably on the edges between white and full-on pels.
One way this is achieved is by clamping the output to zero for input values (V) less than the matrix threshold value (T) for that pels position (modulo the width and height of the threshold matrix) minus Delta1 (D1), i.e., Output=0 if V<(T−D1). The output is forced to the maximum intensity for input values more than the matrix threshold value plus Delta2 (D2), i.e., Output=15 if V>(T+D2). For values between the threshold minus D1 and plus D2, a lookup table (LUT) allows arbitrary assignment of the intensity within the four-bit per color capability of the printer. These intermediate values usually occur near either white or fully saturated pels. Separate Lookup Tables (LUTs) are accessed for the even and odd pel positions due to the parallelism in the hardware. For threshold matrices with an even width, independent LUTs are used for the even and odd columns of the threshold matrix. Still, conventional wisdom asserts that 300 lines per inch (lpi) screens are not possible. Furthermore, conventional wisdom also asserts that 300 lpi screens are bad for the printer and add to its wear.
It can be seen that there is a need for a halftone method and apparatus that provides a line screen frequency of N/2 for a printer resolution of N without negative print effects.